The present invention relates to ink jet printing and particularly to a method and apparatus for sensing and for correcting certain types of errors in the operation of an ink jet printer.
Continuous ink jet printers are based on stimulated formation of the ink drops from a continous ink jet filament at a rate determined by an external perturbation source. The ink drops are selectively charged and deflected according to an external data source such that ink drops emitted from the nozzle of the printing head selectively impinge on a substrate and generate a printing or marking pattern on it.
The charges carried by the drops are defined by the field to which the filament is subject at the moment of drop break-off from the jet filament. Typically, the ink is conductive, and the jet filament functions as an electrode which provides the charges necessary to charge the drops. The external charging field is typically provided by close-by electrodes in a capacitive arrangement relative to the jet filament.
Continuous ink jet printers are divided into two types of systems: binary, and multi-level. In binary systems, the drops are either charged or uncharged and accordingly either reach or do not reach the substrate at a single predetermined position. In multi-level systems, the drops can receive a large number of charge levels and accordingly can generate a large number of print positions.
The process of drop formation depends on many factors associated with the ink rhelogy (viscosity, surface tension), the ink flow conditions (jet diameter, jet velocity), and the characteristics of the perturbation (frequency and amplitude of the excitation). Typically, drop formation is a fast process, occurring in the time frame of a few microseconds. However, because of possible variations in one or more of the several factors determining the drop formation, there are possible variations in the exact timing of the drop break-off. These timing variations, which can be described by phase shifts in the period of drop break-offs, can cause incorrect charging of drops if the electrical field responsible for drop charging is turned-on or turned-off (or changed to a new level) during the drop break-off itself. Therefore it is necessary to keep the data pulse in-phase relative to the drop break-off timing, in order to obtain accurate drop charging and printing.
Previous continuous ink jet systems which contain a typical nozzle diameter of 35-70.mu. operate at relatively high drop generation frequencies, typically higher than 60 kilohertz. Therefore, the drop period is small, in the order of 15 microseconds, and the drop formation time corresponds to about 20% or more of the drop cycle. This indicates that phase control in continuous ink jet systems has to be very tight in order to guarantee correct operation continuously.
Many techniques for phase control have been devised. Some drops are cyclically or constantly monitored for the value of charge they carry by using sensitive electrometers. These electrometers are prone to EMI and RFI interference; and because of the need to place them very close to the stream of drops, serious maintenance problems might develop.
In multi-jet systems, the use of electrometer based phase sensing for each jet in the head becomes extremely difficult and costly. Therefore, techniques were devised to overcome phasing problems which are not based on direct sensing of drop charges, but rather which are based on the design and/or direct sensing of the excitation signal itself. However, these techniques were also found to be extremely complicated and also only partially accurate particularly with ink printers having a large number of nozzles.
Examples of known systems are described in U.S. Pat. Nos. 4,590,483, 5,408,255 and 5,502,474.